The present invention relates to a sliding structure for an automotive internal combustion engine that has excellent friction characteristics and high durability. More specifically, the present invention relates to a sliding structure for use in an automotive engine valve train or intake/exhaust system in which the sliding portion of a sliding member is coated with a specific hard carbon film to attain a low friction coefficient and high durability, reliability and seizure resistant.
A valve train and an intake/exhaust system of an automotive internal combustion engine include various sliding members, such as a cam lobe and its counterpart member, i.e., a valve lifter or a shim attached to an end face of the lifter. Above all, the sliding friction between the cam lobe and the lifter accounts for 20% of the total engine mechanical loss when the engine runs at low speeds (including idling speed). The technique of reducing the friction between the cam lobe and the lifter is important to provide a direct improvement of vehicle fuel efficiency. Herein, the surface pressure developed by sliding contact between the cam lobe and the lifter is among the highest in the internal combustion engine, and the lubrication between the cam lobe and the lifter is temporarily cut off due to their sliding mechanism. It can be thus said that the lubrication state of the cam lobe and the lifter is extremely severe. There are some conceivable effective ways to reduce the sliding friction between the cam lobe and the lifter. For example, the sliding portions of both the cam lobe and the lifter can be smoothed to improve the lubrication state and thereby reduce direct contact (metal contact) between the cam lobe and the lifter. A solid lubricant or lubricant additive may be used to reduce the friction on metal contact between the cam lobe and the lifter. In view of the foregoing, it is proposed to smooth the sliding portion of the lifter, and then, coat the sliding portion with a hard thin film of titanium nitride (TiN) or chromium nitride (Cr2N) or a resinous material containing therein a solid lubricant e.g. molybdenum disulfide (MoS2).
The greatest merit of a hard thin film formed by a physical vapor deposition (PVD) method or a chemical vapor deposition (CVD) method is its surface hardness much higher than that attained by surface treating (e.g. plating) or surface hardening (e.g. heat treatment). By the application of such a hard thin film to a sliding member, the wear resistance of the sliding member can be significantly improved to prevent a deterioration in surface roughness under lubrication. Also, the sliding member can be prevented from wearing away its counterpart member due to the deterioration in surface roughness. It is thus possible to keep the sliding friction from increasing owing to an increase in the direct contact (metal contact) between the sliding member and the counterpart member and possible to maintain the initial lubrication state over an extended time period. In addition, it is expected that the application of the hard thin film to the sliding member would provide a smoothing effect on the counterpart member such that the sliding portions of both the sliding member and the counterpart member can be smoothed for improvement of the lubrication state.
Among various hard carbon films, amorphous carbon films such as a diamond-like carbon (DLC) film are known for high hardness and solid-lubricant-like ability to provide a low friction coefficient with no lubricant. In microscopic analysis, the sliding contact between the sliding member and the counterpart member under lubrication includes a region in which the sliding member makes sliding contact with the counterpart member via a lubricant film and a region in which the sliding member and the counterpart member make direct contact (metal contact) between their surface roughness peaks. With the expectation that the sliding friction between the sliding member and the counterpart member can be reduced under no lubrication, the application of the DLC film to the sliding member, notably the region in which the sliding member makes direct contact with the counterpart member, has been recently examined as a low-friction technique for the internal combustion engine.
However, the hard carbon film formed by the PVD or CVD method has higher internal stress and hardness than that formed by surface treating (e.g. plating). In the application of such a hard carbon film to the sliding member, there arise a problem that the hard thin film often becomes separated from the base portion of the sliding member and/or becomes cracked.
In order to prevent the separation of the hard carbon film, it is proposed to provide an intermediate layer so as to improve the adhesion between the film and the base, and to form the film in a multilayer structure to lessen the film internal stress. On the other hand, there are few proposals of preventing the cracking of the hard carbon film and the separation of the hard carbon film resulting from the film cracking by controlling the surface roughness and profile of the hard thin film as well as the surface roughness and profile of the counterpart member. One of such proposals is to control the surface roughness of the cam lobe and the lifter shim (Japanese Laid-Open Patent Publication No. JP 11-294118A). This proposal is based on the principle that the input of load to the film can be controlled by limiting the roughness of the cam lobe and the lifter shim to a given value or smaller. Another proposal is to control the surface profile of the hard carbon film, more specifically to control the height and amount of macro particles (droplets) remaining on the surface of the film formed by arc ion plating (Japanese Laid-Open Patent Publication No. JP7-118832A).